diff --git a/test/sync_architecture_hardening_test.dart b/test/sync_architecture_hardening_test.dart
new file mode 100644
index 0000000..5335be6
--- /dev/null
+++ b/test/sync_architecture_hardening_test.dart
@@ -0,0 +1,110 @@
+import 'dart:convert';
+import 'dart:typed_data';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:http/http.dart' as http;
+import 'package:http/testing.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:motorcycle_dash/services/sync/r2_upload_service.dart';
+import 'package:motorcycle_dash/services/sync/sync_models.dart';
+import 'package:motorcycle_dash/services/sync/sync_outbox.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  group('Sync outbox hardening', () {
+    test('interrupted uploading item is requeued after restart', () async {
+      SharedPreferences.setMockInitialValues({
+        'motorsignal_sync_outbox_manifest_v1': jsonEncode([
+          SyncItem(
+            localRideId: 42,
+            providerId: 'firebase',
+            status: SyncStatus.uploading,
+            createdAt: DateTime(2026, 9, 1),
+          ).toJson(),
+        ]),
+      });
+      final prefs = await SharedPreferences.getInstance();
+      final outbox = SyncOutbox(prefs);
+      expect(outbox.getItem(42, 'firebase')?.status, SyncStatus.queued);
+    });
+
+    test('duplicate enqueue remains one logical provider/ride item', () async {
+      SharedPreferences.setMockInitialValues({});
+      final prefs = await SharedPreferences.getInstance();
+      final outbox = SyncOutbox(prefs);
+      await Future.wait([
+        outbox.enqueue(7, 'firebase'),
+        outbox.enqueue(7, 'firebase'),
+      ]);
+      expect(outbox.count, 1);
+      expect(outbox.getItem(7, 'firebase')?.idempotencyKey, 'firebase:7');
+    });
+
+    test('malformed persisted identity is ignored without killing the queue', () async {
+      SharedPreferences.setMockInitialValues({
+        'motorsignal_sync_outbox_manifest_v1': jsonEncode([
+          {'localRideId': -1, 'providerId': '', 'status': 'queued', 'createdAt': 'bad'},
+          SyncItem(localRideId: 8, providerId: 'firebase', createdAt: DateTime(2026, 9, 1)).toJson(),
+        ]),
+      });
+      final prefs = await SharedPreferences.getInstance();
+      final outbox = SyncOutbox(prefs);
+      expect(outbox.count, 1);
+      expect(outbox.getItem(8, 'firebase'), isNotNull);
+    });
+  });
+
+  group('R2 gateway hardening', () {
+    const validHash = '0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef';
+
+    test('valid response requires a user-scoped object key', () async {
+      final client = MockClient((request) async => http.Response(
+            jsonEncode({'objectKey': 'users/u1/rides/ride_1_20260901_120000/telemetry.zip', 'size': 4}),
+            200,
+          ));
+      final service = R2UploadService(client: client, workerBaseUrl: 'https://worker.test');
+      final result = await service.uploadTelemetryBundle(
+        idToken: 'token',
+        remoteRideId: 'ride_1_20260901_120000',
+        zipBytes: Uint8List.fromList([1, 2, 3, 4]),
+        sha256Hash: validHash,
+        appVersion: '1.0.2',
+      );
+      expect(result.success, isTrue);
+    });
+
+    test('invalid identifiers are rejected before network I/O', () async {
+      var requests = 0;
+      final client = MockClient((request) async {
+        requests++;
+        return http.Response('{}', 200);
+      });
+      final service = R2UploadService(client: client, workerBaseUrl: 'https://worker.test');
+      final result = await service.uploadTelemetryBundle(
+        idToken: 'token',
+        remoteRideId: '../escape',
+        zipBytes: Uint8List.fromList([1]),
+        sha256Hash: validHash,
+        appVersion: '1.0.2',
+      );
+      expect(result.success, isFalse);
+      expect(result.isRetryable, isFalse);
+      expect(requests, 0);
+    });
+
+    test('rate limiting is retryable and delegated to SyncManager backoff', () async {
+      final client = MockClient((request) async => http.Response('rate limited', 429));
+      final service = R2UploadService(client: client, workerBaseUrl: 'https://worker.test');
+      final result = await service.uploadTelemetryBundle(
+        idToken: 'token',
+        remoteRideId: 'ride_1_20260901_120000',
+        zipBytes: Uint8List.fromList([1]),
+        sha256Hash: validHash,
+        appVersion: '1.0.2',
+      );
+      expect(result.success, isFalse);
+      expect(result.isRetryable, isTrue);
+    });
+  });
+}